Storage structures for storing silage, grain, or other materials frequently employ a bottom unloader to unload the stored material. The bottom unloader is mounted for rotation at the center of the foundation of the structure and is adapted to dislodge the stored material as it rotates in the structure and convey the dislodged material to the center of the structure where it is delivered to a radially extending trough formed in the foundation of the structure. With the storage of non-free flowing materials, such as silage, the bottom unloader may include a cutter chain that travels in an endless path on the cutter arm as the arm rotates within the silo. With free flowing material, such as grain, the bottom unloader may take the form of an auger which rotates about the structure and simultaneously rotates about its own axis. The auger acts to dislodge and convey the stored material toward the center of the structure where it is deposited in the trough and conveyed to the exterior.
In the past the unloaders have been driven by a motor located on the outside of the structure and a drive mechanism interconnects the motor with the inner end of the sweep arm at the center of the storage structure. Intermittent motion can be provided for the arm by a ratchet and pawl mechanism, as disclosed in U.S. Pat. No. 2,635,770. As shown in the aforementioned patent, an overload protection device can also be incorporated in the drive in the event the sweep arm encounters an obstruction which prevents rotation of the sweep arm.